My experience is that the Non-Project investments have limitations in their use if you are trying to detail and track functions performed while providing the 'Service'. By that I mean, if you take the IT Management view of a Service, the investments would be set up for 'Services' such as WAN, Cloud Storage, WINTEL, Hosted Storage, Mobile Telephone, Desktop SOE, etc.
Within each of these Services you may want to track operations around Problems, Incidents, Changes, etc (see ITIL Service Operation | IT Process Wiki ). The lack of a WBS for the 'Service' investment greatly hinders the ability to track resource time / effort related to the operations of the Service.
Further more, DevOps is changing the nature of who Service Operations are being managed. Particularly when you are looking at Management Reporting requirements such as how much time is spent on enhancing the Service (Dev) versus keeping the lights on (Ops) and where is the focus of the Service in the Service Management lifecycle.
In the end, on my previous implementation we registered all of our 'Services' as Project investments. But what we requested the Service Managers to do on BAU / Run type Service investments was very different to what we expected the Project Managers to do on a full blown Project. But this decision meant that we had to focus on configuration enhancements that allowed us to delineate between the different types of investments. The thing that was consistent was how we managed Cost Plans / Team Plans.
The 'Other Work' investment was reserved for truly non-work activities such as General Leave, Public Holidays, Training, Sick Leave, etc.